Fix detection of postroll when seeking to duration

Also mark all ads as played when the postroll plays, in the case the
player is backgrounded then resumed and the user seeks back.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=164262738

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaAdsLoader.java

@@ -572,16 +572,26 @@ public final class ImaAdsLoader implements Player.EventListener, VideoAdPlayer,
       return;
     }
     if (!playingAd && !player.isPlayingAd()) {
-      long positionUs = C.msToUs(player.getCurrentPosition());
-      int adGroupIndex = timeline.getPeriod(0, period).getAdGroupIndexForPositionUs(positionUs);
-      if (adGroupIndex != C.INDEX_UNSET) {
+      checkForContentComplete();
+      if (sentContentComplete) {
+        for (int i = 0; i < adPlaybackState.adGroupCount; i++) {
+          if (adPlaybackState.adGroupTimesUs[i] != C.TIME_END_OF_SOURCE) {
+            adPlaybackState.playedAdGroup(i);
+          }
+        }
+        updateAdPlaybackState();
+      } else {
+        long positionMs = player.getCurrentPosition();
+        timeline.getPeriod(0, period);
+        if (period.getAdGroupIndexForPositionUs(C.msToUs(positionMs)) != C.INDEX_UNSET) {
           sentPendingContentPositionMs = false;
-        pendingContentPositionMs = player.getCurrentPosition();
+          pendingContentPositionMs = positionMs;
         }
-      return;
       }
+    } else {
       updateImaStateForPlayerState();
     }
+  }
 
   @Override
   public void onPlaybackParametersChanged(PlaybackParameters playbackParameters) {
@@ -672,8 +682,8 @@ public final class ImaAdsLoader implements Player.EventListener, VideoAdPlayer,
   }
 
   private void checkForContentComplete() {
-    if (contentDurationMs != C.TIME_UNSET
-        && player.getCurrentPosition() + END_OF_CONTENT_POSITION_THRESHOLD_MS >= contentDurationMs
+    if (contentDurationMs != C.TIME_UNSET && pendingContentPositionMs == C.TIME_UNSET
+        && player.getContentPosition() + END_OF_CONTENT_POSITION_THRESHOLD_MS >= contentDurationMs
         && !sentContentComplete) {
       adsLoader.contentComplete();
       if (DEBUG) {

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/SinglePeriodAdTimeline.java

@@ -23,7 +23,7 @@ import com.google.android.exoplayer2.util.Assertions;
 /**
  * A {@link Timeline} for sources that have ads.
  */
-public final class SinglePeriodAdTimeline extends ForwardingTimeline {
+/* package */ final class SinglePeriodAdTimeline extends ForwardingTimeline {
 
   private final long[] adGroupTimesUs;
   private final int[] adCounts;